1. Field of the Invention
The present invention relates to a rubber composition and to a pneumatic tire using the rubber composition, and more particularly, to a rubber composition and to a pneumatic tire using the rubber composition in which wet skid resistance and grip performance on a dry road surface are excellent and abrasion resistance can be improved.
2. Description of the Related Art
Heretofore, silica has been used for the rubber composition of a tire so as to improve the performances thereof.
For example, Japanese Patent Application Laid-Open (hereinafter, "JP-A") No. Sho-63-270751 and JP-A No. Sho-64-9248 disclose a rubber composition compounded with a predetermined amount of silica in order to obtain a high performance tire.
Further, JP-A No. Hei-3-252431, JP-A No. Hei-3-252433, and JP-A No. Hei-3-25431 disclose a pneumatic tire in which a rubber composition compounded with silica, silane coupling agent, and a specific polymer is used for a tread in order to improve wet skid resistance, rolling resistance, and abrasion resistance.
Several other tread rubber compositions compounded silica are proposed in order to improve the performances of a tire (e.g., JP-A No. Hei-4-224840, JP-A No. Hei-5-271477, JP-A No. Hei-5-51484, JP-A No. Hei-7-48476, and the like).
However, silica particles tend to cohere together due to hydrogen bonding of silanol groups which are functional groups on the surfaces of the silica particles. For improving the dispersion of silica particles into rubber, the mixing time must be increased. When dispersion of silica particles into rubber is insufficient, a problem arises in that processability in processes such as extrusion and the like deteriorates due to the increase in the Mooney viscosity.
Moreover, the surfaces of the silica particles are acidic. Therefore, there are problems in that basic substances used as vulcanization accelerators are absorbed such that vulcanization is not carried out sufficiently, and a sufficient modulus of elasticity is not obtained.
In order to solve these problems, various types of silane coupling agents have been developed. For example, use of a silane coupling agent as a reinforcing material is described in Japanese Patent Application Publication (hereinafter, "JP-B") No. Sho-50-29741. However, the use of a silane coupling agent as a reinforcing material is still insufficient for improving fracture properties, workability, and processability of a rubber composition to high standards. Rubber compositions in which a combination of silica and silane coupling agent is used as a reinforcing material are described in JP-B No. Sho-51-20208 and others. However, this method of using a combination of silica and silane coupling agent as a reinforcing material has a drawback in that flow of the uncured compounded rubber is markedly inferior and workability and processability deteriorate, although reinforcement of the compounded rubber can be remarkably improved and fracture properties are improved.
The drawbacks of the conventional technologies in which silane coupling agents are used arise due to the following mechanism. When the mixing temperature of rubber is low, the silanol group on the surface of the silica does not react sufficiently with the silane coupling agent, and as a result, the sufficient reinforcing effect is not obtained. Moreover, some of the alcohol formed by the reaction of the silanol group on the surface of the silica and the silane coupling agent does not vaporize completely during mixing because of the low mixing temperature, and the residual alcohol in the rubber vaporizes during an extrusion process so as to form blisters.
On the other hand, when mixing is conducted at high temperatures of 150.degree. C. or more, the silanol group on the surface of the silica and the silane coupling agent sufficiently react with each other, and as a result, the reinforcing property is improved. Dispersion of the silica into the rubber is also improved, a rubber composition having a good abrasion resistance is obtained, and the formation of blisters in an extrusion process is suppressed. However, in this temperature range, gelation of the polymer caused by the silane coupling agent takes place simultaneously, and the Mooney viscosity markedly increases. Thus, processing in later stages becomes impossible in actuality.
Therefore, when a silane coupling agent is used in combination with silica, a multistep mixing must be conducted at a temperature lower than 150.degree. C., and marked decrease in productivity is inevitable. When the mixing is conducted at a low temperature, dispersion of silica and carbon black into the rubber is insufficient and abrasion resistance deteriorates.